1. Field of the Invention
The present invention relates to a temperature compensated oscillator, its manufacturing method, and an integrated circuit for a temperature compensated oscillator, and particularly relates to a temperature compensated oscillator for correcting a change in frequency, due to a circumferential temperature of the oscillator, by digital control, its manufacturing method, and an integrated circuit for temperature compensated oscillator.
2. Description of the Related Art
In various kinds of communication devices such as a portable telephone, etc., a system clock having higher frequency accuracy is desired at present as communication speed is increased. Therefore, a temperature compensated crystal oscillator is used. FIG. 4 shows such a temperature compensated crystal oscillator. This temperature compensated crystal oscillator is constructed by externally attaching a crystal resonator XL to an integrated circuit for a temperature compensated crystal oscillator. The integrated circuit for the temperature compensated crystal oscillator is constructed by a varicap diode CV for adjusting an oscillating frequency of an oscillator circuit 41, a temperature detector 42, an A/D converter 43 for A/D-converting an output of the temperature detector 42, a non-volatile memory 44 for storing compensating data for temperatures experienced by the temperature compensated crystal oscillator, a correcting value signal generating circuit 45, and a D/A converter 46. The correcting value signal generating circuit 45 generates a correcting value signal on the basis of the compensating data address-designated by an output of the A/D converter 43 and read from the non-volatile memory 44. The D/A converter 46 D/A-converts the correcting value signal, and generates a control voltage of the varicap diode CV.
Temperature characteristics of the frequency accuracy of a crystal resonator (AT cut) generally used in a temperature compensated oscillator circuit can be approximated by a cubic function represented as follows.
xcex94f/f=A3(Txe2x88x92T0) 3+A1(Txe2x88x92T0)+A0
T0 shows a reference temperature, and is different in accordance with an individual crystal together with a coefficient of the cubic function. For example, there are temperature characteristics of the frequency accuracy as shown in FIG. 3.
The oscillating frequency of the crystal oscillator circuit is provided as follows.
f0=fs(1+1/(2C0/C1(1+CL/C0))
Here, fs, C0 and C1 respectively show a resonant frequency of the crystal, an equivalent parallel capacity and an equivalent series capacity, and CL shows a load capacity of the oscillator circuit. It should be understood from this formula that temperature compensation can be carried out by adjusting the frequency if CL can be changed in accordance with temperature T. The varicap diode is used as the equivalent series capacitor CL.
The compensating data are set as follows in such a temperature compensated crystal oscillator. The oscillator is really constructed by connecting a crystal oscillator to the integrated circuit for temperature compensated crystal oscillation, and is arranged within a constant temperature bath. Then, a voltage is applied from the exterior to the varicap diode CV every setting of a predetermined temperature. A switch 47 of FIG. 4 is connected to the side of a terminal A in advance. An oscillating output signal of the oscillator is monitored, and a control voltage Vc of the varicap diode for obtaining a predetermined frequency is specified, and characteristics of the D/A converter 46 at that temperature are measured. Each coefficient (A3, A1, A0) of the cubic function and data for adjustment of the D/A converter 46 are calculated on the basis of these measuring data, and are stored to the non-volatile memory 44 as the compensating data corresponding to each temperature.
At the actual operating time, the compensating data are read in a state in which data obtained by A/D-converting analog temperature information detected by the temperature detector 42 are set to an address of the non-volatile memory 44. These compensating data are read and outputted to the correcting value signal generating circuit 45 so that a correcting value signal is generated. This correcting value signal is D/A-converted by the D/A converter 46 so that the control voltage Vc of the varicap diode CV is generated. The switch 47 of FIG. 4 is connected to the side of a terminal B, and the control voltage Vc is applied to the varicap diode CV.
In the conventional method, the compensating data are calculated by so-called off-line processing in which the measuring data at each temperature are once stored to an external device and are separately processed. Thereafter, the compensating data are written to the non-volatile memory. Therefore, it takes much cost and labor.
Further, temperature characteristics of the temperature detector 42, the A/D converter 43, the D/A converter 46 and the varicap diode CV, and frequency-temperature characteristics of the crystal resonator XL are individually different from each other. These different temperature characteristics are corrected by approximate values so that it is difficult to improve combination accuracy.
It is also very difficult to reduce cost and improve compensation accuracy since absolute exactness of temperature setting is required to extract the compensating data.
Therefore, an object of the invention is to provide a temperature compensated oscillator for which it is unnecessary to accurately set and detect temperature at a manufacturing time, and reduce manufacture cost of the temperature compensated oscillator.
In the invention, a temperature compensated oscillator has a frequency comparing circuit for comparing the frequency of an oscillating output signal of it and the frequency of an external reference frequency signal externally inputted, and also has a register for determining each bit on the basis of results of this comparison. A digital signal from the register is supplied to a D/A converter for generating a control voltage of a variable capacity element for adjusting the frequency of the oscillating output signal. A self compensating operation for sequentially determining each bit of the register on the basis of the comparing results of every comparison, and for conforming the frequency of the oscillating output signal to the frequency of the external reference frequency signal is provided. When the frequency of the oscillating output signal is conformed to the frequency of the external reference frequency signal, the digital signal from the register is set as compensating data corresponding to a detected temperature of a temperature detector at that time. The compensating data are determined by performing the self compensating operation every predetermined temperature change detected by the temperature detector. Thus, off-line processing is removed, and it is possible to reduce cost and labor for controlling an absolute temperature at a manufacturing time.
Further, combination accuracy can be also improved by correcting the characteristics of all elements in total.
Further, proper compensating data can be extracted by self-detecting a temperature change at a setting time of the compensating data so that no strictness of temperature setting is required. Therefore, it is expected that total cost performance of an adjustment is greatly improved.
A temperature compensated oscillator of the invention comprising a temperature detector for outputting an analog signal according to temperature, an A/D converter for converting the analog signal from the temperature detector to a digital signal, a memory from which compensating data are read using the digital signal from the A/D converter as an address, a D/A converter for converting the compensating data from the digital signal to the analog signal, a variable capacity element set by the analog signal from the D/A converter as a control voltage, an oscillator circuit for generating an oscillating output signal by oscillating a resonator such as a crystal resonator and a surface acoustic wave resonator, by using the variable capacity element as a frequency adjusting element of the oscillating output signal, a frequency comparing circuit for comparing frequencies of the oscillating output signal and an external reference frequency signal externally inputted, a register for sequentially determining a value of each bit of the register on the basis of frequency comparing results of the frequency comparing circuit, and a switching circuit for selectively supplying the compensating data read from the memory and the digital signal outputted from the register to the D/A converter; wherein the oscillator circuit performs an oscillating operation using the digital signal outputted from the register to the D/A converter through the switching circuit; the frequency of the oscillating output signal is changed by sequentially determining the value of each bit of the register on the basis of the comparing results of the frequency comparing circuit every comparing operation; the frequency of the oscillating output signal of the oscillator circuit and the frequency of the external reference frequency signal are conformed to each other; and the digital signal outputted from the register at a time when the frequency conforming occurs is written to the memory as the compensating data corresponding to a detecting temperature of the temperature detector every predetermined temperature step in a state in which the digital signal outputted from the A/D converter in accordance with the detecting temperature at that time is set as an address.
The invention also resides in a manufacturing method of a temperature compensated oscillator which comprises; a process for constructing a temperature compensated oscillator using a temperature detector for outputting an analog signal according to temperature, an A/D converter for converting the analog signal from the temperature detector to a digital signal, a memory from which compensating data are read using the digital signal from the A/D converter as an address, a D/A converter for converting the compensating data from the digital signal to an analog signal, a variable capacity element set using the analog signal from the D/A converter as a control voltage, an oscillator circuit connected to a resonator such as a crystal resonator, and generating an oscillating output signal by performing an oscillating operation according to the resonator, and using the variable capacity element as a frequency adjusting element of the oscillating output signal, a frequency comparing circuit for comparing frequencies of the oscillating output signal and an external reference frequency signal externally inputted, a register for sequentially determining a value of each bit on the basis of frequency comparing results of the frequency comparing circuit, and a switching circuit for selectively supplying the compensating data read from the memory and the digital signal outputted from the register to the D/A converter; a process for operating the temperature compensated oscillator at a predetermined specific temperature; a process for inputting the external reference frequency signal to the frequency comparing circuit; a process in which the oscillator circuit performs an oscillating operation by supplying the digital signal outputted from the register to the D/A converter through the switching circuit, and the frequency of the oscillating output signal is changed by sequentially determining the value of each bit of the register on the basis of the comparing results of the frequency comparing circuit every comparing operation, and the frequency of the oscillating output signal of the oscillator circuit and the frequency of the external reference frequency signal are conformed to each other; a process for writing the digital signal outputted from the register at a time when the frequency conforming occurs to the memory as the compensating data corresponding to a detecting temperature of the temperature detector in a state in which the digital signal outputted from the A/D converter in accordance with the detecting temperature at that time is set to an address; and a process for writing the compensating data required within a predetermined temperature compensating range to the memory by changing a circumferential temperature of the temperature compensated oscillator at a predetermined speed from the specific temperature so as to cross the predetermined temperature compensating range, and performing the writing operation of the compensating data every predetermined temperature step. The specific temperature preferably lies outside the temperature compensating range.
An integrated circuit for temperature compensated oscillation is also preferably constructed by integrating each construction of the temperature compensated oscillator except for the resonator in one chip.
In the temperature compensated oscillator, its manufacturing method and the integrated circuit for temperature compensated oscillator, it is also preferable that the temperature step corresponds to a predetermined changing width of the detecting temperature of the temperature detector, and the temperature step is partitioned when the digital signal outputted from the A/D converter shows the predetermined changing width. It is also preferable that the temperature step is narrowly set in a temperature range in which a temperature fluctuation of frequency accuracy of the oscillator becomes large. It is further preferable that the resonator is a crystal resonator, and the variable capacity element is a varicap diode.